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Full text of "Text Book Of Mechanical Engineering"

102                                 Welding.

resorted to, the crystallised portion will be left weak and little
better than cast iron. This should be carefully noted in making
connecting rods of steam engines, or indeed any article the break-
ing of which might cause danger to life.

Welding.ŚWrought iron cannot be cast,* but it can be
welded without difficulty; that is, it may be joined piece to piece
by heating and hammering, and work of great intricacy may thus
be formed. The welding temperature for wrought iron is reached
at about 2800░ Fahrenheit, and the two pieces to be welded are
heated to this temperature, which is detected by the iron beginning
to throw out sparks. Two points have to be noticed. The iron
should be, if possible, drawn out so that a scarf may be made,
when welded; this is shewn at A, Fig. 103, and, as will be seen, a
greater surface for welding is thereby presented. But, if it be
drawn out too fine, it will burn away when put into the fire for
the welding heat, and to prevent this it should be left rather thick
at the ends, as at B ; the lump may be easily levelled afterwards.
The two pieces to be welded should both be at their proper heat
at the same time, which the smith ensures by changing their
positions in the fire, so as to advance the one or retard the other.
Withdrawing, he sprinkles them with sand, which forms a siliceous
film or flux, and prevents scale by oxidation. Putting them now
together, the smith gives one or two blows to fix them, and he and
the striker then finish by rapid alternating blows. If the flux be
carefully expelled and the joint well hammered while hot, the bar
will be nearly as strong there as at any other section. Borax is
used as a flux in steel welding. (See Appendices /. and Iff.,
pp. 748 and 917.)

The scarf weld is the one most commonly practised, but the
fork weld at c, Fig. 103, is often introduced for large work on
account of its greater security.

Having thus briefly mentioned the operations of heating and
welding, we shall now proceed to describe the forging of a few
objects.

The making of a Bolt with hexagonal head is shewn in
Fig. 104. A round bar A is taken, of suitable length; it is
heated at one end, and jumped or upset, namely, is lifted by

* See note at end of Chapter III.